Wavelength power equalization by attenuation in an optical switch

ABSTRACT

A method for equalizing optical signal power in a group of optical signals transmitted through an optical switch in an optical transmission system. In one embodiment a group of optical signals is input into an optical switch having at least one movable mirror array with a plurality of reflectors formed thereon, the optical beam being directed onto a selected at least one reflector and wherein attenuating the optical beam is accomplished by controllably detuning at least one of the selected at least one reflector to attenuate the optical beam.

TECHNICAL FIELD

[0001] The invention described herein relates to methods and apparatusfor achieving selective power attenuation in the optical signals passingthrough optical switches. In particular, such selective attenuation isused to accomplish optical power balancing (or equalization) among theoptical signals passing through optical switches. More particularly, theinvention relates to power balancing using optical switches of fiberoptic transmission systems to attenuate optical power in selectedchannels of an optical system such that the power distribution of theoptical channels falls within some user selected power margin.

BACKGROUND

[0002] As is well known, fiber optic technology is a rapidly growingfield with vastly expanding commercial applicability. As with alltechnologies, fiber optic technology is faced with certain practicaldifficulties. In long haul optical transmission systems, optical signalpower loss causes unpredictable but significant losses in signalstrength. Such losses are caused by a variety of factors, including butnot limited to, variations in optical path length, equipmentcharacteristics, environmental conditions, the effects of aging, and soon. In view of these and other factors, it has proven difficult tomaintain optical signals at relatively uniform power levels as they passthrough the optical transmission system. This is particularly so as theoptical signals pass through the optical switches of an opticaltransmission system.

[0003] Another factor which contributes to the development ofnon-uniform signal power in a group of many optical signal channels isrelated to the need for continuing signal amplification of the opticalsignals as they negotiate an optical path through an opticaltransmission system. Optical amplification is required to avoidexpensive electrical signal regeneration over optical paths that canextend over thousands of miles. The chain of amplifiers arranged alongan optical path is referred to as a cascade of amplifiers. A problemwith such cascades of amplifiers is that optical amplifiers have astrong wavelength dependence on gain. This means that the amplifiersamplify optical signal at some wavelengths more than optical signals atother wavelengths. This and other problems induce non-uniformity inoptical signal. More troubling, the effects of amplifier gainnon-uniformity increase with each amplifier in the cascade of opticalamplifiers along the optical path. This means that as the optical signalpasses through each amplifier, the effects of amplifier gainnon-uniformity intensify. Therefore, the longer the signal path, thegreater the number of amplifiers, and as a result, the greater thedegree of gain non-uniformity. In addition, the buildup of optical noisefrom the amplifier gain peaks can quickly saturate a cascade ofamplifiers.

[0004] When groups of optical signals (channels) having non-uniformoptical power pass through switching nodes a variety of problems canoccur. One problem of particular significance is the possibility ofmisdirecting light from one optical channel onto another opticalchannel. As result, an input signal (or a portion thereof) from onechannel is output into the wrong output fiber. Consequently, a signalintended for one output fiber can be contaminated by signal intended foranother output fiber. This phenomenon is referred to herein as“cross-talk”. This problem is magnified to a distressing level insituations where the optical signal power in one or more fibers issignificantly greater than the optical power in other fibers. Forexample, if a first optical beam is 10 dBm more powerful than anadjacent second optical beam, if even a small fraction of the light fromthe first beam is deflected into the path of the second beam, the signalof the second beam will be corrupted by the cross-talk from the firstbeam. Moreover, as optical switch size steadily decreases, the marginfor error in switching systems also decreases. As a result, in systemswith non-uniform optical power levels in the fibers, the likelihood ofcross-talk and the resulting problems significantly increases.

[0005] One conventional approach for addressing gain non-uniformityproblems is through the use of static gain equalization using commercialfilters, such as fiber Bragg gratings. In such implementation, thewavelength dependent loss related to the filters correspondsapproximately to the wavelength dependent gain from optical amplifiers.However, optical amplifier gain is affected by other factors, such as,input signal level, temperature, and amplifier aging effects. As aresult, simple Bragg gratings do not provide a satisfactory solution togain non-uniformity problems.

[0006] Numerous other approaches toward solving signal non-uniformity inoptical network applications have been tried. Although some of theseapproaches work well enough in some situations, each suffers from itsown set of limitations. All require the addition of new hardware whichintroduces new causes for signal loss in the system. Additional newhardware increases system complexity, thereby increasing unreliability.Moreover, these hardware systems all increase cost. Therefore, there arecontinuing efforts to provide improved methods and apparatus forreducing the effects of non-uniform optical power in opticaltransmission systems without adding new hardware, without substantiallyincreasing system complexity and unreliability, and without increasingcost. Method and apparatus embodiments constructed in accordance withthe principles of the present invention are intended to solve these andother problems.

SUMMARY OF THE INVENTION

[0007] In accordance with the principles of the present invention, anapparatus and method for achieving a more uniform optical powerdistribution among a group of optical channels by attenuating opticalpower in the selected optical channels to provide improved signalbalancing across the many channels of a group of optical channels isdisclosed.

[0008] One embodiment comprises a method for equalizing optical signalpower in a group of optical signals transmitted through an opticalswitch in an optical transmission system by inputting a group of opticalsignals into an optical switch, defining a user selected power range,and attenuating selected optical signals in the group of optical signalssuch that the signal power of each optical signal in the group ofoptical signals falls within the user selected power range.

[0009] A further embodiment comprises inputting a group of opticalsignals into an optical switch, monitoring the optical power of thegroup of optical signals, determining the optical power of the weakestsignal of the group of optical signals to define a baseline opticalpower level which is implemented in combination with a user selectedpower margin to define a user selected power range into which selectedoptical signals are attenuated such that the signal power of eachoptical signal in the group of optical signals falls within the userselected power range.

[0010] Further embodiments include embodiments where the user selectedpower margin is freely adjustable by a system user.

[0011] Other embodiments implement user selected power margins of about2 dBm, 1.5 dBm, 1 dBm, and 0.5 dBm.

[0012] Yet another embodiment comprises inputting a group of opticalsignals into an optical switch which includes at least one movablemirror array having a plurality of reflectors and wherein the group ofoptical signals is directed onto the reflectors, monitoring the opticalpower of the group of optical signals, defining a baseline optical powerlevel based on the optical power of the group of optical signals,implementing a user selected power margin in combination with thebaseline optical power level to define a user selected power range, andselectively attenuating the optical power of signals which fall outsidethe user selected power range by controllably detuning selectedreflectors to attenuate the selected optical signals such that thesignal power of each optical signal in the group of optical signalsfalls within the user selected power range.

[0013] Other aspects and advantages of the invention will becomeapparent from the following detailed description and accompanyingdrawings which illustrate, by way of example, the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The following detailed description of the embodiments of theinvention will be more readily understood in conjunction with theaccompanying drawings, in which:

[0015]FIG. 1 is a block diagram of optical transmission system of thepresent invention.

[0016]FIG. 2 is a block diagram of an embodiment of an opticalcross-connect (switch) of the present invention.

[0017] FIGS. 3(a) and 3(b) are graphs depicting optical power in a groupof optical channels before and after optical power equalization inaccordance with the principles of the present invention.

[0018] FIGS. 4(a), 4(b), and 5 are graphs depicting optical power in agroup of optical channels which are subject to varying power levelsduring optical power equalization in accordance with the principles ofthe present invention.

[0019]FIG. 6 is an illustration of a portion of an optical switchembodiment capable of practicing the principles of the present invention

[0020]FIG. 7 is an illustration of a MEMS reflector embodiment that canbe used in accordance with the principles of the present invention.

[0021]FIG. 8 is an illustration of a portion of another optical switchembodiment capable of optical power balancing in accordance with theprinciples of the present invention

[0022] FIGS. 9A-9B are end-on views of optical fibers and optical beamswhich illustrate aspects of beam misalignment.

[0023]FIG. 10 is a flowchart describing a suitable method embodiment foraccomplishing optical signal balancing in an optical transmission systemin accordance with the principles of the present invention.

[0024]FIG. 11 is a three-dimensional graphical depiction of a powerprofile in an optical channel with optical power being plotted on thevertical axis versus reflector control parameters on the horizontalaxes.

[0025]FIG. 12 is a illustration describing a suitable dither patterncapable of use in detuning reflectors to attenuate an optical signal(beam) in accordance with the principles of the present invention.

[0026]FIG. 13(a) shows a 3-D power profile in an optical channelgraphically depicting the effects of dither on optical power when anoptical beam is detuned in accordance with the principles of the presentinvention.

[0027]FIG. 13(b) is a two dimensional depiction of a portion of thegraph of FIG. 13(a) showing the dither effect in accordance with theprinciples of the present invention.

[0028]FIG. 14 is a depicts a simplified single axis dither patterncapable of use in detuning reflectors to attenuate an optical signal(beam) in accordance with the principles of the present invention.

[0029] It is to be understood that in the drawings like referencenumerals designate like structural elements.

DETAILED DESCRIPTION OF THE DRAWINGS

[0030] The present invention has been particularly shown and describedwith respect to certain preferred embodiments and specific featuresthereof. The embodiments set forth herein below are to be taken asillustrative rather than limiting. It should be readily apparent tothose of ordinary skill in the art that various changes andmodifications in form and detail may be made without departing from thespirit and scope of the invention.

[0031] The embodiments described below provide methods and apparatus forachieving power balancing using optical switches of fiber optictransmission systems to attenuate optical power in selected channels ofan optical system such that the power distribution of the opticalchannels falls within some chosen power margin.

[0032]FIG. 1 is a block diagram of an optical transmission system 1 thatemploys optical cross-connect switches 2, 4, 6, 8, 10, and 12. Theoptical cross-connect switches 2, 4, 6, 8, 10, and 12 allow the opticaltransmission system 1 to route data traffic from a variety of differentsources to a variety of different target destinations. Each opticalcross-connect switch 2, 4, 6, 8, 10, 12 branches to numerousdestinations allowing quick and versatile switching to route datatraffic to a desired destination. Moreover, such systems 1 recover fromfailures relatively quickly. For example, if the optical fiber line 20connecting switches 4 and 6 is accidentally severed, the data carried inoptical path 3 cannot reach switch 8. However, in the event of such afailure, the optical cross-connect switch 2 redirects the optical pathfrom path 3 to path 5 avoiding the cut in fiber line 20 enabling thedata to reach switch 8.

[0033] The optical transmission system 1 can carry digital data, voicesignals, and video signals over fiber optic lines at varioustransmission speeds. The transmission system 1 can send digitalinformation in various formats, for example, in ATM format. Such anoptic system 1 can send internet and intranet traffic. The opticaltransmission network 1 can, for example, use ISDN, DWDM, or WDMtechnologies to transfer digital information at extremely highcapacities.

[0034]FIG. 2 is a block diagram of an optical cross-connect switchembodiment 100. A plurality of input optical signals (or channels) (S₁,S₂, . . . , S_(n−1), S_(n)) 116 are input into a switching element 30.The switching element 30 of the switch 100 redirects the input signalsas needed and plurality of output optical signals (or channels)(S₁′,S₂′, . . . , S′_(n−1), S_(n)′) 117 are output from the switch 100. Asthey exit the switching element 30 the power level of the output opticalsignals (S₁′, S₂′, . . . , S′_(n−1), S_(n)′) is detected with opticaldetector elements 136. Such monitoring is continuous and ongoing as theoptical channels are constantly adjusted by the control circuitry of thecontrol element 36. These adjustments reflect, among other things,changes in optical paths, changes in signal power, changes in theoperational status in the fibers within the system, as well as a list ofother factors. Typically, as each optical signal exits the switchingelement 30, a detector 136 measures the optical power of the signals,and each optical signal of the group of optical signals (S₁′, S₂′, . . ., S′_(n−1), S_(n)′) is optimized for maximum optical power. As statedpreviously, optical paths are continuously changing to redirect opticalsignals to accommodate a variety of system needs. As a result, signalmonitoring and signal optimization is conducted at periodic intervals.Moreover, after optimization each signal in the group of optical signals(S₁′, S₂′, . . . , S′_(n−1), S_(n)′) will not have the same opticalpower as the other signals in the group. Thus, it is common to have anon-uniform optical power distribution in a group of optical signals(S₁′, S₂′, . . . , S′_(n−1), S_(n)′) as it is output from a switch 30.

[0035]FIG. 3(a) graphically illustrates seven sample optical signals(S₁′, S₂′, S₃′, S₄′, S₅′, S₆′, and S₇′) at seven different power levelsas they are output from switching element 30. In reality the group ofoptical signals can include 100's, 1000's, or even more optical signals.The optical power of each output signal (S₁′, S₂′, S₃′, S₄′, S₅′, S₆′,and S₇′) is measured by the detectors 136. FIG. 3(a) shows a powerdistribution between the strongest (S₁′) and weakest (S₇′) signals inthe group.

[0036] The inventors contemplate that embodiments of the presentinvention can selectively attenuate the optical power of selectedoptical signals to achieve a desired degree of power equalization amonga group of optical signals. An important aspect of the embodiments isthe achievement of an equalized power distribution in a group of opticalchannels such that each optical channel falls within a user selectedpower range. Moreover, such achievement of an equalized powerdistribution in a group of optical channels is accomplished withoutarbitrarily limiting the output power of any output channel. Inachieving this goal there are two parameters which must be consideredwhen implementing aspects of the present invention, desired degree ofpower equalization (also referred to as power balancing) and optimalsignal power. It is desirable to have the greatest possible signal powerand also desirable to have the distribution of signal power in the groupof optical signals be as small as possible. Thus, the weakest signal(here, S₇′) applies a baseline level (lower limit) of optical power inthe group of optical signals being attenuated. Of course it is possibleto attenuate the signal strength of the group of optical signals to alevel below its baseline level, it is just not the most preferredapproach.

[0037] With continuing reference to FIG. 3(a) it can be seen that theoptical power of the weakest channel (S′₇) is about 7 dBm. Thus, abaseline level is 7 dBm. In perfect signal balancing each of the opticalchannels (S′₁, S′₂, S′₃, S′₄, S′₅, S′₆ and S′₇) are selectivelyattenuated until they each have a power level of 7 dBm. However, thislevel of signal balancing is not required, satisfactory opticalperformance can be achieved by attenuating selected channels such thatan “equalized” power distribution is formed wherein the optical power ofeach channel of the of the group of optical channels falls within someuser selected power margin. A preferred power margin is 1 dBm. Otheruseful power margins include, but are not limited to 2.0 dBm, 1.5 dBm,and 0.5 dBm. The user selected margin and the baseline power define auser selected power range. Thus, in the depicted example, the powerlevels of each of the optical channels should fall within the userselected power range of between 7 dBm and 8 dBm. However, an importantfeature of some embodiments of the invention is that the optical powermargin can be freely adjusted by the user (such margins can be dictatedby network tolerances, the application, etc.). The power margin can benarrower or wider depending on the needs of the system or user. Thispower margin can be set remotely or locally using whatever criteria theuser deems important. Additionally, the power margin can be adjusteddepending on system needs based on non-local system requirements (e.g.based on system needs many hundreds of miles away).

[0038] Once the power range is set, each optical channel having a powerlevel falling outside the range is attenuated to a level inside therange. FIG. 3(b) shows the group of optical channels (S′₁, S′₂, S′₃,S′₄, S′₅, S′₆ and S′₇) after selectively attenuating the optical powerin the stronger channels (S′₁, S′₂, S′₃, S′₄, S′₅, S′₆ and S′₇) untilthey all fall within a preferred 1 dBm power margin 150.

[0039] Moreover, the optical power of the channels are maintained withinthe user selected power margin 150 by periodic monitoring and poweroptimization/attenuation which forms part of the embodiment. FIGS. 4(a),4(b), and 4(c) show how embodiments of the invention can adapt to theconstant variations in optical power among the group of opticalchannels. FIG. 4(a) shows an “equalized” power distribution similar tothat of FIG. 3(b). In the example of FIG. 4(b), the weakest channel(S′₇) undergoes a loss of optical power (shown by the arrow).Consequently, the optical power of channel S′₇ falls outside the userselected power margin 150. Such power variations are common in opticalsystems and arise from a wide range of sources. For example, in thedepicted example, power in the weakest channel (S′₇) drops from 7 dBm to6.7 dBm. This defines a new baseline power level of 6.7 dBm. This isdetected by the periodic monitoring of optical power in the system.Using the same the user selected power margin of 1 dBm, the power rangeis changed from 7-8 dBm to 6.7-7.7 dBm. The optical channels havingoptical power in excess of 7.7 dBm are controllably attenuated to bringthe channels back into compliance with the user selected optical powermargin, as shown in the example of FIG. 4(c). Alternatively, if thebaseline power level remains stable, but some other channels increase inoptical power to a level falling outside the user selected power marginthe non-compliant channels are selectively attenuated until they fallwithin the user selected power range. In another situation, if thebaseline power level remains stable, and the optical power in at leastone other channel decreases to a level falling below the baseline powerlevel, the new weakest channel defines a new baseline power level. As aresult, the lower end of the user selected power range is changed. Theother channels are selectively attenuated such that the optical power ofall channels falls within the new user selected power range. In yetanother situation, if the baseline power level increases, each of thechannels is analyzed and weakest channel defines a new increasedbaseline power level that changes the lower end of the user selectedpower range. The other channels are selectively attenuated such that theoptical power of all channels fall within the new user selected powermargin. What is common to each of these situations is that the channelhaving the lowest optical power defines the baseline power level towhich the user selected power margin is calibrated. And the opticalchannels are equalized at or above the baseline power level (that is tosay selectively and controllably attenuated until the optical power ofeach channel falls within the user selected power range).

[0040] Referring again to FIG. 2, the optical cross-connect switch 100includes a switch element 30 and optical detector elements 136, whichare coupled to a control element 36. The switch element 30 is shown ingreater detail in FIGS. 6 and 8. Optical channels are input into theswitch element 30 through an array of input fiber optic cables (S₁, S₂,. . . , S_(n−1), S_(n)) 116. Additionally, the optical channels areoutput from the switch element 30 into an array of output fiber opticcables (S₁′, S₂′, . . . , S′_(n−1), S_(n)′) 117.

[0041] Control circuitry of the control element 36 is coupled to theswitch element 30. Such control circuitry 36 can comprise a wide rangeof devices or combinations of devices. In one embodiment the controlelement 36 includes digital signal processors (“DSP's”) and memory whichcontrol the operation of the switch 30. Examples of satisfactory DSP'sare TMS320C6211 digital signal processors manufactured by TexasInstruments of Dallas, Tex. The control element 36 controls MEMSreflectors (e.g., 112, 113 of FIG. 6) of switch element 30. The controlelement 36 receives optical power values from optical detector elements136 (see FIG. 2), which are associated with switch embodiments of thepresent invention. One satisfactory optical power detecting element 136is a fiber optic power splitter (e.g., a Model #9-107798-2 produced byAMP of Harrisburg, Pa.) and a photodetector element, such as a #ETX75photodetector produced by Epitaxx of West Trenton, N.J. In thealternative, an integrated optical tap/photodetector could be used toaccomplish both tasks, for example, a #IPD-10 produced by Santec ofJapan. Optical power information received by the detector 136 issupplied to the control circuitry 36 where it is used to determinereflector orientation.

[0042] The control element 36 is able to generate mathematicalapproximations of the relationship between optical power versus MEMSreflector parameters (e.g., reflector position, voltage applied toreflector actuators, or other measurable and controllable parametersindicative of reflector orientation) based on optical power values,obtained by the detectors 136, and received by the control element 36.The control element 36 orients the MEMS reflectors of the switch 30based on mathematical approximations of the relationship between opticalpower versus reflector position. The control element 36 accomplishesthis, for example, by running code stored in memory.

[0043] The control circuitry of the control element 36 can include aprocessor which is also coupled to the switch 30. In one embodiment, asatisfactory processor is an MPC860 Power PC microprocessor supplied byMotorola, Inc. of Schaumburg, Ill. In conventional application theprocessor operates to maintain optimal signal paths through switch 30.In the present embodiments, the processor also enables the detuning ofthe reflectors in the switch 30 to selectively and controllablyattenuate light beams in order to accomplish the optical powerequalization discussed above.

[0044]FIG. 6 illustrates a switching element embodiment 30 which can beused in accordance with the principles of the present invention. Theswitching element 30 includes an input fiber block 110, into which aplurality of input optical fibers 116 are fitted and aligned with thelenses 111A of a lens array 111. The switching element 30 also includesan output fiber block 115, into which a plurality of output opticalfibers 117 are fitted and aligned with the lenses of a lens array 114.These components direct input light beams onto a pair of mirror arrays(which are also referred to herein as movable mirror planes) 112, 113where the light beams are directed to desired output optical fibers 117.Each of the mirror arrays 112, 113 has a plurality of electricallyaddressable mirror and frame assemblies (reflectors) 200. As previouslydiscussed, the orientation of the reflectors 200 is controlled by thecontrol element 36. An example of a reflector 200 is shown in theexpanded view shown in FIG. 6 and in greater detail in FIG. 7.

[0045] Optical channels enter the switching element 30 through the inputoptical fibers 116 as light beams (for example, the single beam B beingshown here). The light beams are passed through the input lens array111, where in preferred implementation, the optical beams are collimatedby the lenses 111A of the input lens array 111 before being introducedto the mirror arrays 112, 113 where they are reflected into the desiredoutput optical fibers 117. Typically the light beams are collimated asthey pass through the lenses of the output lens array 114 into theoptical fibers 117 of an output fiber array 115 where the light beams Bare passed on through the remainder of the optical transmission system(not shown).

[0046] By moving the reflectors 200 of the input mirror array 112 (firstinput mirror array) and the reflectors of output mirror array 113(second input mirror array), light beams B from one input fiber may beswitched from one fiber to another. Moreover, the reflectors can beoriented to maximize optical power in the beams B. Reflectors aretypically, made up of a micro-electro-mechanical combination of mirror,frame, gimbels, and actuators. Such systems are referred to asmicro-electro-mechanical systems (MEMS). These MEMS reflectors 200 canbe constructed by a variety of means. One preferred example is set forthin U.S. patent application Ser. No. 09/471,796 entitled, “AMicromachined Reflective Arrangement” which is hereby incorporated byreference, for all purposes. Each mirror array includes a plurality ofthese MEMS reflectors 200.

[0047]FIG. 7 depicts an example of a MEMS reflector assembly 200. EachMEMS reflector assembly 200 includes a mirror element 220 and a frameelement 221. The mirror element 220 is rotatably suspended in the frameelement 221 by gimbels 223. And the frame element 221 is rotatablysuspended in a mirror array (e.g., 112, 113) by gimbels 224. Each MEMSreflector 200 can be rotated about two axes (shown here as being rotatedby the mirror gimbels 223 about an x-axis and by the frame gimbels 224about a y-axis). Such rotation alters the optical path of beamsreflected by the reflectors 200. Although the mirrors 220 depicted hereare generally elliptical in shape, there is no requirement that they beso. Moreover, although the axes about which the reflectors rotate aredepicted as orthogonal to each other, there is no requirement that theybe so.

[0048]FIG. 8 illustrates another embodiment of an optical cross-connectswitch 31 which can be used to achieve power balancing in a group ofoptical signals. Here, a single array of input fiber optic cables 116 isintroduced to a block 110 and lens array 111. Only a single mirror array112 is required due to the presence of a fixed mirror 118, which directsthe optical beams B back to the mirror array 112, back through the lensarray 111, and back into the “input” fiber array 110. In thisembodiment, the fiber array 110 operates as both an input and outputfiber array. The embodiment shown in FIG. 8 has the advantage ofallowing light that passes through any fiber to be switched to any otherfiber in the fiber array 116. The embodiment shown in FIG. 8 can also beused to practice the principles of the present invention. Additionally,although the invention is explained with reference to the embodiments ofFIGS. 6 and 8 (i.e., embodiments having a single mirror array 112 or twomirror arrays 112 and 113), the inventors contemplate that the presentinvention can be practiced using alternative embodiments having anynumber of mirror arrays.

[0049] In order to explain certain aspects of the present inventionreference is made to FIGS. 9A and 9B. The need to selectively attenuateoptical power in selected optical channels has already been explained.By attenuating an optical beam as it passes through an optical switch,selective attenuation of optical power in an optical channel may beachieved. The following discussion will help illustrate aspects of suchattenuation. FIG. 9A is an edge-on depiction of an unattenuated opticalbeam B and a corresponding optical fiber 117. In conventionalimplementation, the unattenuated beam B is reflected by the reflectorsof a switch such that maximum optical power is transmitted and the beamB is positioned to enter the output fiber in a substantially centeredmanner. FIG. 9B shows a beam B which has been attenuated in accordancewith an embodiment of the present invention. The beam is offset withrespect to the corresponding optical fiber 117. This offset is inresponse to the detuning of reflectors in the optical switch. The degreeof such detuning determines the amount of attenuation of the opticalbeam B.

[0050] As previously stated, in order to achieve a power distribution ina group of optical channels which falls within a user selected powermargin, selected channels are controllably attenuated to equalizeoptical power in the group of optical channels. Preferred embodimentsachieve this by intentionally misaligning selected optical beams suchthat the optical paths of the beams do not perfectly align to theintended output fibers of the output fiber array. Such misalignmentattenuates optical power in selected beams and the degree of attenuationdepends on the amount of signal loss required to achieve a more balancedoptical power distribution in the optical channels.

[0051] Referring again to the block diagram of FIG. 2, attenuation isaccomplished by detuning the reflectors in the switch 30. The controlelement 36, analyzing information provided by the optical detectors 136,controllably and selectively “detunes” the reflectors to achieveattenuation in selected optical channels. When reflectors are orientedsuch that maximum beam power is produced, the reflectors are said to be“tuned”. In the described embodiments, the operation of the controlcircuitry and actuator elements which orient the reflectors (e.g.,electrostatic actuators) are altered from their normal tunedorientation. Instead, selected reflectors are detuned, thereby reducingthe optical power transmitted by those beams. The embodiments of thepresent invention balance optical power in a plurality of opticalsignals (e.g., a group of optical channels) by increasing optical pathloss in the stronger channels to provide signal balancing across themany channels of a group of optical channels.

[0052]FIG. 10 is a flowchart illustrating a method embodiment 1000 foroperating optical cross-connects (switches) in an optical transmissionsystem in a manner that achieves equalized power distribution among agroup of optical channels such that the optical power of each opticalchannel falls within a user selected power range. The method 1000 beginsby inputting a plurality of optical channels into an optical switch(Step 1001). These optical channels are typically comprised of opticalsignals output from the fibers of the optical transmission system. Suchoptical signals can be digital data signals, audio signals, videosignals, or any other optically transmittable signal known to thosehaving ordinary skill in the art. Such signals can be transmitted usingISDN, WDM, DWDM, or other optical data transmission modes. Examples ofpreferred switches are shown in FIGS. 6 and 8.

[0053] The optical power in each of the optical channels is detected andmeasured (Step 1003). In one embodiment the optical power is detectedand measured by an optical power detecting element 136 (See, FIG. 2) asit is output from the switch element. Such detecting and measuring is anongoing process conducted at periodic intervals.

[0054] A group of optical channels is selected by the user (Step 1005).Such selection is typically in response to some predetermined criteria.For example, the criteria can be that each channel in the group ofchannels must include only working beams and test beams can be selectedout. Alternatively, all optical channels passing through the switch canbe selected as the group of optical channels. The optical power marginis also selected (Step 1007). For example, an optical power margin of1.0 dBm can be selected.

[0055] The measured values for optical power in each channel of thegroup of optical channels is used to determine the baseline power valuefor a group of optical channels (Step 1009) (e.g., the lowest opticalpower in a group of optical channels). Once a baseline power value isdetermined, the user selected power margin is used to determine a userselected power range (Step 1011). The purpose of the user selected powermargin is to allow the user to define the equalization tolerance of thesystem by selecting a power margin (e.g., 1 dBm). Moreover, the userselected power range allows the user to define the power range intowhich the optical power of each optical channel in a group of opticalchannels must fall. The degree of attenuation required in each opticalchannel to bring that optical channel to a power level which fallswithin the user selected power margin is then calculated (Step 1013). Inone embodiment, such determinations and calculations are performed bythe control element (e.g., 36, FIG. 2). Based on the measured opticalpower of the optical channels and a determination of the user definedpower range the amount of attenuation required for each optical channelcan be determined. Once the desired amount of attenuation is determinedfor each optical channel, each channel is attenuated as needed to fallwithin the user selected power margin (Step 1015). Typically, theattenuation is accomplished by iteratively adjusting the reflectorpositions until a beam of desired power is achieved. Alternatively, thereflectors are iteratively adjusted until a relationship between opticalpower and reflector position can be determined, at which point thereflectors are adjusted such that the desired power is achieved in thebeam. The attenuated and equalized group of optical channels is outputfrom the switch (Step 1017). As a result, the optical power in eachoptical channel is such that the group of optical channels has a moreuniform power distribution.

[0056] In a simplified example, an input group of optical channelshaving power values of 10 dBm, 7 dBm, 6 dBm, and 4 dBm, respectively,are measured at an optical switch. The weakest channel has an opticalpower level of 4 dBm. Thus, the baseline power level is determined as 4dBm. If the user selected power margin is 1 dBm then the desired powerrange is 4-5 dBm. The group of optical channels will be selectivelyattenuated and output as a group of optical channels having a moreuniform power distribution (i.e., power values between 4 dBm and 5 dBm).For example, the 10 dBm, 7 dBm, and 6 dBm optical channels areattenuated until their optical power values fall within the 4 dBm to 5dBm range. Continuing the example, the attenuation of the 10 dBm, 7 dBm,and 6 dBm optical channels is determined to be −5 dB to −6 dB, −2 dB to−3 dB, and −1 dB to −2 dB respectively. The described embodimentsattenuate selected optical signals by the controlled detuning ofreflectors. The details of such attenuation are described in greaterdetail hereinbelow. The now equalized group of optical signals areoutput from the switch and can be transmitted elsewhere in the opticaltransmission system.

[0057] A Preferred Method for Achieving Controlled Loss of Optical Powerin an Optical Beam

[0058] Implementations of the invention use controlled detuning of thereflectors in an optical switch to selectively attenuate a group ofoptical beams in order to achieve a more uniform distribution of opticalpower in the group of optical beams. A preferred method embodiment forachieving optical beam attenuation is disclosed hereinbelow. It shouldbe noted that other methods can be used for controlling optical signalattenuation. Moreover, such methods can be varied depending on theoptical switch configuration used.

[0059] Referring, for example, to the embodiment of FIG. 6, thereflectors 200 are oriented to optimize beam intensity. The optimalreflector orientations can be stored in a look-up table which forms partof the control circuitry 36. However, because such optical systems aredynamic, this information is constantly changing and is constantlyupdated using information from the optical detectors 136. A suitablemethod and apparatus for dynamically managing this optimal beamintensity is set forth in U.S. patent application Ser. No. 09/586,711,filed on Jun. 5, 2000, entitled “Positioning a Movable Reflector in anOptical Switch”. The forgoing patent application is hereby incorporatedby reference.

[0060] From this optimal orientation the selected reflectors aredeliberately detuned to reduce optical beam intensity. The attenuationmethod embodiment disclosed herein performs calculations which willgenerate an appropriate degree of attenuation in each selected fiber.Moreover, the method may be applied to each reflector in the opticalpath of the selected beam, but advantageously, need only be applied toone reflector in the optical path.

[0061] The graph of FIG. 11 depicts a 3-D optical power profile 1100 inan optical channel. In an optical switch the optical power of an opticalbeam can be depicted as a three-dimensional curve 1100 with themagnitude of optical power on one axis (shown here as the log of opticalpower along the Z-axis), and reflector parameters or beam position onthe other two axes (the X- and Y-axes). The reflector parameters caninclude a wide range of variables. For example the X- and Y-axes canrefer to an optical beam position on an output fiber. In the depictedexample, the beam power is maximized (shown by point 1101) when the beamis centered in the output fiber. As can be seen from the graph, if thebeam drifts from the optimal position, beam optical power is reduced.

[0062] Alternatively, the power profile 1100 can be a graph of beamoptical power versus reflector parameters. Such reflector parameters caninclude, but are not limited to, the physical orientation or position ofthe reflector (e.g., the angle of the reflector) or the voltage appliedto the actuators that move and position the reflectors. In applying theprinciples to the graph of FIG. 11, for example, the X-axis canrepresent the angle of deflection of a reflector about one axis. And,the Y-axis can represent the angle of deflection of a reflector aboutanother axis. The power profile peak (at point 1101) will be at thereflector angles which reflect the maximum amount of power into theoutput fiber. In another embodiment, the X-axis can represent the amountof voltage applied to an actuator that moves the reflector about oneaxis. And, the Y-axis can represent the amount of voltage applied to anactuator moving the reflector about another axis. The peak power valueP_(max) (at point 1101 on the graph) will be at the actuator voltageswhich position the reflectors such that a maximum amount of beam opticalpower enters the output fiber. In switch embodiments having both inputreflectors and output reflectors for each beam, a power profile can beformed for each input reflector and each output reflector. Either, orboth of these power profiles can be used to attenuate the optical beamin accordance with the principles of the present invention.

[0063] The depicted power profile 1100 approximates a Gaussian curve.Point 1100 represents peak or optimum optical power P_(max) in thechannel. When the optical power in an output channel is at P_(max) thereflector (or combination of reflectors) are at the best or optimalorientation for transmitting a light beam at maximum power (i.e., thereflector(s) are “tuned”). Optical power at other, less optimal examplereflector orientations 1102, 1103, 1104 are also shown for comparison.Optical power falls off as the reflector orientation is detuned from theoptimal orientation.

[0064] It can be seen that the optical channels can be attenuated byapplying controlled detuning of the reflectors with respect to eitherthe X- or Y-axes. In one embodiment, a combination of both the inputreflectors and the output reflectors are used to attenuate the opticalbeam. For example, one reflector is tuned to reflect maximum opticalpower in the beam and the other reflector is used to attenuate the beam.The power profiles 1100 can illustrate this, for example, the beam isadjusted (by moving the reflectors) such that the power valuesrepresented by a first power profile 1100 remain at a maximum powervalue. Additionally, the beam is adjusted (by moving the reflectors)such that the power values represented by a second power profile remainat the peak (optimized) power value in one axis (e.g., the X-axis) whilethe power is attenuated by detuning the reflectors in the other axis. Inthis way the optical power in the optical beams can be controllablyattenuated by selectively detuning reflectors. For example, thereflectors at the input reflector can be oriented to optimize beam powerand the output reflectors can be selectively detuned by optimizing theoutput reflector position in one axis but detuning the output reflectorin the other axis. Other approaches are possible but it is frequentlysimplest to detune only one reflector in one dimension while keeping theother reflector dimensions in an optimized orientation.

[0065] The following discussion integrates the above process into thegeneral process of achieving an equalized power distribution in a groupof optical signals (as in Steps 1001-1017 of FIG. 10). Once the opticalpower in each signal in a group of optical signals is measured (Step1003) and the user selected power range is determined (Steps 1007-1011),then the required degree of optical power attenuation for each opticalchannel is determined (Step 1013). Such a calculation determines therequired power P_(req) for each channel so that each channel fallswithin the user selected power range.

[0066] Once the required power P_(req) for each beam in a group of beamsis determined, and the degree of attenuation required to achieve therequired power P_(req) is determined, the beams are attenuated (Step1015). One suitable method for attenuating the optical power in anoptical beam is detailed hereinbelow. Each optical beam subject toattenuation is moved through a “dither” pattern. Each beam is dithered(moved in small distances) about a start position to obtain a series ofpower readings which are used to attenuate the beam.

[0067]FIG. 12 shows an example of one suitable dither pattern. Theoptimally focused beam starts at position 250. Beam power P₀ at position250 is measured taken using detector (136 of FIG. 2). The beam isdithered about the start point 250 in a dither pattern 270, whichincludes dither points 250 through 254. The beam is dithered to the foursurrounding positions 251, 252, 253, and 254 where power measurementsP_(+y), P_(+x), P_(−y), and P_(−x) are made. For increased accuracy manypower readings can be taken at each point 250-254 and their resultantvalues integrated. For example, 100 or 1000 separate measurements can betaken. These values are integrated and averaged to provide accuratepower measurements.

[0068] One procedure for moving a beam through a dither pattern isdescribed. The beam begins at the start point 250, power P₀ is measuredhere. A reflector is detuned such that the beam moves an initial ditherdistance 264 in the positive y-direction to a point 251 where a powerreading P_(+y) is made. The reflector is then detuned such that the beammoves a dither distance in the y-direction and a dither distance in thex-direction 265 to point 252 where another power reading P_(+x) is made.Again, the reflector is detuned such that the beam moves a ditherdistance in the x-direction and a dither distance in the y-direction toposition 253 where another power reading P_(−y) is made. Again, the beamis dithered or moved in the x- and y-directions to a point 254 whereanother power reading P_(−x) is made. The initial dither distance whichthe optical beams are dithered in the x- and y-directions (264, 265) canbe on the order of the physical dimensions of the mirror arrays and thereflectors contained therein. For example, a dither movement on theorder of {fraction (1/10)}^(th) of a mirror width is a suitable initialdither distance.

[0069]FIG. 13 shows a power profile similar to that of FIG. 11. A beamis shown having an initial power P₀ (i.e., the power reading taken atoptimized beam position 250 of FIG. 12) at point 1301. The beam isdithered to dither point 251 (of FIG. 12) and the power value P_(+y) ismeasured. The P_(+y) is graphed (at point 1302) with respect to thechange in position in the +Y direction. The power value P_(+x) isgraphed (at point 1303) with respect to dither point 252. The powervalue P_(−y) is graphed (at point 1304) with respect to dither point253, and finally, the power value P_(−x) is graphed (at point 1305) withrespect to point 254.

[0070] As indicated above, although the x and y axes of FIG. 13 aredepicted as physical distances, such axes can refer to other measurableand controllable parameters related to reflector detuning, for example,positive and negative voltages applied to the actuators controlling thereflector orientation. In such a case, the optical power profiles graphoptical power versus x and y actuator voltages.

[0071] As has already been indicated, the optical beam is controllablymoved through the dither pattern by the control circuitry 36. Referringto the embodiment of FIG. 6, either the output reflectors 113 or theinput reflectors 112 or both the output reflectors 113 and the inputreflectors 112 can be moved to implement the dither pattern. For thesake of simplicity, the following discussion refers to a preferredembodiment which implements the dither pattern using the only thereflectors of the output mirror array 113. As this is beingaccomplished, the control circuitry 36 is receiving optical powerinformation from the optical detector 136 for positions 250 through 254.

[0072] Once the power values (e.g., P₀, P_(y), P_(x), P_(−y), P_(−x))are obtained for each dither point, they are evaluated to determine thedegree of reflector detuning needed to attenuate each beam. Referring toFIG. 13(a), it can be seen that P_(req) lies on the power profile curvebetween P₀ (shown as point 1301 of FIG. 13(a)) and P_(+y) (shown aspoint 1302 of FIG. 13(a)). The beam can be dithered with respect to theY-axis to obtain a plot of power values versus position in the Y-axis.An example of such a two dimensional plot is shown in FIG. 13(b). Basedon this information, a new position can be calculated such that the beamhas a power value at or near P_(req). This estimated power value P_(est)can be used as a basis for a new dither pattern wherein the beam isdithered a smaller distance and the calculations are used to obtain amore accurate position. In this way beams can be adjusted until the beampower is P_(req) and the beam is successfully attenuated. The beam isaccurately attenuated using such iterative processes. Methods likelinear interpolation or other more accurate estimation methods can beused to more accurately model beam power versus reflector positionbehavior. This process is repeated for each optical channel needingattenuation until each channel achieves an optical power that fallswithin the user selected power range.

[0073] In one embodiment, the reflectors 200 of both the input array andthe output array can be detuned in the x- and y-directions to achieveattenuation. This presents a sophisticated four-dimensional problem.Alternatively, either the input reflectors or the output reflectors canbe detuned to attenuate the beam. This simplifies the problem by detunedonly a single reflector in the x- and y-axes to create a two-dimensionalproblem.

[0074] In other so-called two-D switch embodiments (also sometimesreferred to as digital or ON/OFF switches) the optical beam isattenuated by detuning only one reflector. Such two-D embodiments arediscussed in detail in “Electrostatic Micro Torsion Mirrors for anOptical Switch Matrix” (Journal of Microelectromechanical Systems, Vol5, No. 4, December 1996), which is hereby incorporated by reference.Advantageously, in such embodiments, the reflector need be detuned inonly one axis (e.g., the x-axis). This eliminates many of thecomplexities inherent in trying to successfully attenuate the beam intwo axes simultaneously.

[0075]FIG. 14 illustrates a single axis (here the x-axis) ditherpattern. An initial power value P₀ is determined at the start point 250.The reflector is detuned to dither the beam in the positive x-directionto dither point 252 which lies some initial dither distance dd_(i) awayfrom the start point 250. An example of a suitable initial ditherdistance dd_(i) is again {fraction (1/10)}^(th) of a mirror width. Oncethe beam is dithered to dither point 252, a power reading P_(+x) istaken. The beam can be dithered a distance in the negative x-directiontoward point 254 (e.g., to a point −dd_(i) from the start 250) where apower reading of P_(−x) is obtained. In a manner similar to thatpreviously discussed the beam can be attenuated to P_(req). Again, theprocedure is repeated for each channel needing attenuation.

[0076] The present invention has been particularly shown and describedwith respect to certain preferred embodiments and specific featuresthereof. However, it should be readily apparent to those of ordinaryskill in the art that various changes and modifications in form anddetail may be made without departing from the spirit and scope of theinvention as set forth in the appended claims. In particular, it iscontemplated by the inventors that the principles of the presentinvention can be practiced with optical switch embodiments having one,two, three, and more movable mirror planes. Further, reference in theclaims to an element in the singular is not intended to mean “one andonly one” unless explicitly stated, but rather, “one or more”.Furthermore, the examples provided herein are intended to beillustrative rather than limiting. The inventions illustrativelydisclosed herein can be practiced without any element which is notspecifically disclosed herein.

We claim:
 1. A method for equalizing optical signal power in a group ofoptical signals transmitted through an optical switch in an opticaltransmission system, the method comprising: inputting a group of opticalsignals into an optical switch; defining a user selected power range;and attenuating selected optical signals in the group of optical signalssuch that the signal power of each optical signal in the group ofoptical signals falls within the user selected power range.
 2. A methodas in claim 1 wherein defining a user selected power range includes:monitoring the optical power of the group of optical signals;determining the optical power of the weakest signal the group of opticalsignals, thereby defining a baseline optical power level; implementing auser selected power margin in combination with the baseline opticalpower level to define the user selected power range; and whereinattenuating selected optical signals in the group of optical signalscomprises attenuating the optical power of signals which fall outsidethe user selected power range such that the signal power of each opticalsignal in the group of optical signals falls within the user selectedpower range.
 3. A method as in claim 2 wherein the user selected powermargin is freely adjustable by a system user.
 4. A method as in claim 2wherein implementing a user selected power margin in combination withthe baseline optical power level to define the user selected power rangeincludes implementing a user selected power margin of about 1 dBm.
 5. Amethod as in claim 3 wherein monitoring the optical power of the groupof optical signals includes: periodically monitoring the optical powerof the group of optical signals; and wherein determining the opticalpower of the weakest signal the group of optical signals includesperiodically adjusting the baseline optical power level based onchanging power values for the weakest signal; and wherein the userselected power range is subject to periodic adjusting based on changingpower values for the baseline optical power level.
 6. A method as inclaim 5, wherein said periodically adjusting the baseline optical powerlevel based on changing power values for the new weakest signal includeslowering the baseline optical power level if the power value for the newweakest signal is less than a previous power value for the weakestsignal; and wherein the adjusting of the user selected power rangecomprises implementing the user selected power margin in combinationwith the baseline optical power level derived from the new weakestsignal to define the user selected power range.
 7. A method as in claim5, wherein said periodically adjusting the baseline optical power levelbased on changing power values for the new weakest signal includesincreasing the baseline optical power level if the power value for thenew weakest signal is greater than a previous power value for theweakest signal; and wherein the adjusting of the user selected powerrange comprises implementing the user selected power margin incombination with the baseline optical power level derived from the newweakest signal to define the user selected power range.
 8. A method forequalizing optical signal power in a group of optical signalstransmitted through an optical switch in an optical transmission system,the method comprising: inputting a group of optical signals into anoptical switch; and attenuating selected optical signals in the group ofoptical signals such that a more uniform power distribution is achievedamong the group of optical signals.
 9. A method as in claim 8 whereinattenuating selected optical signals in the group of optical signalssuch that a more uniform power distribution is achieved among the groupof optical signals includes: monitoring the optical power of the groupof optical signals; determining the optical power of the weakest signalthe group of optical signals, thereby defining a baseline optical powerlevel; implementing a user selected power margin in combination with thebaseline optical power level to define a user selected power range; andselectively attenuating the optical power of signals which fall outsidethe user selected power range such that the signal power of each opticalsignal in the group of optical signals falls within the user selectedpower range.
 10. A method as in claim 9, wherein the optical switchincludes at least one movable mirror array having a plurality ofreflectors and wherein inputting the group of optical signals includesdirecting the group of optical signals onto the reflectors; and whereinselectively attenuating the optical power of signals which fall outsidethe user selected power range is accomplished by controllably detuningselected reflectors to attenuate the selected optical signals such thatthe signal power of each optical signal in the group of optical signalsfalls within the user selected power range.
 11. A method as in claim 10,wherein the at least one movable mirror array comprises a single movablemirror array.
 12. A method as in claim 10, wherein the at least onemirror array comprises at least two movable mirror arrays.
 13. A methodas in claim 12, wherein selectively attenuating of optical signals isaccomplished by controllably detuning selected reflectors on one movablemirror array of the at least two movable mirror arrays.
 14. A method asin claim 12, wherein selectively attenuating of optical signals isaccomplished by controllably detuning selected reflectors on two movablemirror arrays of the at least two movable mirror arrays.
 15. A method asin claim 12, wherein the at least two movable mirror arrays include aninput movable mirror array and an output movable mirror array.
 16. Amethod as in claim 15, wherein selectively attenuating of opticalsignals is accomplished by controllably detuning selected reflectors ofthe input movable mirror array and controllably detuning selectedreflectors of the output movable mirror array.
 17. A method as in claim16, wherein each of said reflectors comprise a mirror element and aframe element and wherein said detuning can be accomplished by detuningone of; the mirror element, the frame element, or both the mirrorelement and the frame element.
 18. A method as in claim 15, whereinvariably attenuating selected optical signals is accomplished byvariably detuning selected ones of the plurality of reflectors of theinput array.
 19. A method as in claim 18, wherein said plurality ofreflectors of the input array comprise a mirror element and a frameelement and wherein said detuning can be accomplished by detuning oneof; the mirror element, the frame element, or both the mirror elementand the frame element.
 20. A method as in claim 15, wherein variablyattenuating selected optical signals is accomplished by variablydetuning selected ones of the plurality of reflectors of the outputarray.
 21. A method as in claim 20, wherein said plurality of reflectorsof the output array comprise a mirror element and a frame element andwherein said detuning can be accomplished by detuning one of; the mirrorelement, the frame element, or both the mirror element and the frameelement.
 22. A method for attenuating an optical beam transmitted in anoptical switch, the method comprising: inputting an optical beam into anoptical switch having at least one movable mirror array with a pluralityof reflectors formed thereon; and controllably attenuating the opticalbeam in the switch to obtain a reduced optical power level in theoptical beam.
 23. The method of claim 22, wherein inputting the opticalbeam includes directing the optical beam onto a selected at least onereflector; and wherein attenuating the optical beam is accomplished bycontrollably detuning the selected at least one reflector to attenuatethe optical beam.
 24. The method of claim 23, wherein the at least onemovable mirror array comprises a single movable mirror array.
 25. Amethod as in claim 23, wherein the at least one mirror array comprisesat least two movable mirror arrays.
 26. A method as in claim 25, whereindirecting the optical beam onto a selected at least one reflectorincludes directing the optical beam onto a selected reflector of a firstone of the at least two movable mirror arrays and directing the opticalbeam onto a selected reflector of a second one of the at least twomovable mirror arrays.
 27. A method as in claim 26, wherein attenuatingthe optical beam is accomplished by controllably detuning the selectedreflector of the first one of the at least two movable mirror arrays.28. A method as in claim 26, wherein attenuating the optical beam isaccomplished by controllably detuning the selected reflector of thesecond one of the at least two movable mirror arrays.
 29. A method as inclaim 26, wherein attenuating the optical beam is accomplished bycontrollably detuning both the selected reflector of the first one ofthe at least two movable mirror arrays and the selected reflector of thesecond one of the at least two movable mirror arrays.
 30. A method forequalizing optical signal power in a group of optical signalstransmitted through an optical switch in an optical transmission system,the method comprising: inputting a plurality of optical signals into anoptical switch; measuring output the optical power of the plurality ofoptical signals after they are passed through the optical switch;selecting a group of optical signals from among the plurality of opticalsignals in the optical switch; user selecting an optical power margin;determining a power range; and attenuating selected optical signals inthe group of optical signals such that the signal power of each opticalsignal in the group of optical signals falls within the power range.